Re: [PATCH v2 1/3] xen/arm: pci: introduce PCI_PASSTHROUGH Kconfig option

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

On 7/13/23 14:40, Julien Grall wrote:
> Hi Stewart,
> 
> On 07/07/2023 02:47, Stewart Hildebrand wrote:
>> From: Rahul Singh <rahul.singh@xxxxxxx>
>>
>> Setting CONFIG_PCI_PASSTHROUGH=y will enable PCI passthrough on ARM, even 
>> though
>> the feature is not yet complete in the current upstream codebase. The 
>> purpose of
>> this is to make it easier to enable the necessary configs (HAS_PCI, 
>> HAS_VPCI) for
>> testing and development of PCI passthrough on ARM.
>>
>> Since PCI passthrough on ARM is still work in progress at this time, make it
>> depend on EXPERT.
> 
> While preparing the patch for committing, I noticed that HAS_PASSTHROUGH
> will now allow the user to select one of the IOMMU quarantine options.
> 
> There are three of them right now:
>   1. none
>   2. basic (i.e. faulting)
>   3. scratch page
> 
> The latter is unlikely to work on Arm because we don't setup the scratch
> page. AFAIU, for that, we would need to implement the callback
> quarantine_init().
> 
> I would expect 1 and 2 work. That said, I think 1. would behave like 2.
> because on Arm the device should not be automatically re-assigned to
> dom0. I know this is correct for platform device, but will it be valid
> for PCI as well?

In a system with dom0 where the guest is created from the xl toolstack, we rely 
on "xl pci-assignable-add". Upon domain destruction, the device automatically 
gets assigned to domIO. However, there's nothing preventing a user from 
attempting to invoke "xl pci-assignable-remove", which should assign the device 
back to dom0, but it is not automatic.

With dom0less, PCI devices are not deassigned from the domU.

> Overall, before enabling HAS_PASSTHROUGH, I think there would be a bit
> of tweaking necessary to at least prevent a user to select the option 3
> (either via Kconfig or the command line). And possibly update the
> Kconfig for IOMMU_QUARANTINE to reflect the behavior on Arm.

OK